The present invention relates generally to electrical cable shields and more particularly to extensible electrical cable shields.
Electrical cables, especially those cables used for high speed data transmission, radiate and are susceptible to electromagnetic interference (EMI). One means of prevention of EMI is to enclose such electrical cables in metallic, i.e. highly conductive, shields. The conductive shield, if it supplies the required high conductivity and continuous coverage, will prevent EMI from radiating from the cable.
The requirement for a large capacity of signal distribution in a compact cable has been met with the use of a "ribbon" cable in which a large number, e.g., 50, conductors lie in a single plane and are encased in a common insulating material. An example of such a cable is Scotchflex Model 3365 Cable, manufactured by Minnesota Mining and Manufacturing Company, St. Paul, Minn. This cable provides many signal conductors in a compact cable while affording ease of terminability with mass termination equipment.
One means for constructing a shielded ribbon cable is illustrated by Scotchflex Model 3517 Shielded Ribbon Cable. The shield of this cable comprises an expanded copper mesh, e.g., 4CU6-050 flattened annealed copper foil mesh produced by Delker Corporation, wrapped around the cable. This shield provides the advantages of extensibility and mechanical ruggedness. However, because the mesh is open and is inadequately conductive, its shielding characteristics are marginal or inadequate for many uses.
Another means for shielding a ribbon cable or other cable is to cover the cable with a highly conductive metallic foil such as a copper or aluminum. In one common construction the foil is laminated to a polyester film for reinforcement. However, serious problems occur when using foil shields, particularly when the metallic foil is bonded either to the insulation surrounding the signal conductors or to the inner surface of a jacketing material. A continuous foil shield greatly reduces the flexibility of the cable. Both copper foil and aluminum foil tend to crack when repeatedly flexed. As an example, a continuous one mil thick aluminum foil shield bonded to a 50 mil thick cable core can be expected to show evidence of cracking after the second or third bend around a 3/8 inch diameter mandrel.
Mechanically produced cracks in a ribbon cable usually run transverse to the signal conductors. When using such a cable (a cable with transverse cracks in the shield conductor) in an unbalanced drive situation (a single conductor utilizing a ground return) the shield carries all or part of the return current, the transverse cracks interrupt that current flow resulting in a deleterious effect on cable operation. Cracks enable signal leakage increasing the likelihood of EMI. Even when using such a cable (a cable with transverse cracks in the conductive shield) in balanced drive (a pair of oppositely driven conductors per signal) transverse cracks decrease the shielding effectiveness for common mode (e.g., turn-on pulses and electrostatic discharge sensitivity) and also increases the likelihood of EMI.
The most widely used prior art shield for round cable has been braided wire. When tightly woven and new, a braided wire shield provides high conductivity, high coverage, good to very good shielding and mechanical flexibility and ruggedness. Double layers of braid with silver plating are required for the best shielding performance. Unfortunately, braided wire shields lose effectiveness with age because the connections between wires at cross-overs become unreliable. These conditions are even less certain when a braided shield is woven around a ribbon cable.
Prior art shields have not combined the highly desirable continuous coverage and excellent shielding qualities of metallic foils with the needed flexibility of braided wire.